1. Field of the Invention
The present invention relates to an optical disc apparatus and an optical pickup. More particularly, the present invention relates to an optical disc apparatus and an optical pickup that can accurately locate a fingerprint on an optical disc by detecting a dot-to-dot gap of the fingerprint with a narrow converged beam spot.
2. Description of the Related Art
In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks and the thickness of the recording material film are smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a storage plane actually has a physical dimension in the depth direction, too, the term “storage plane (information plane)” will be replaced herein by another term “information layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or focal point) of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
To get the focus control and tracking control done, a focus error or a tracking error needs to be detected based on the light reflected from an optical disc and the location of the light beam spot needs to be adjusted so as to minimize the error. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal” that are generated based on the light reflected from the optical disc.
Once a fingerprint is left on the surface of an optical disc, the intensity of the reflected light will decrease when the light beam passes through that fingerprint. As a result, the intensity distribution of the light beam will decrease non-uniformly, thus causing a disturbance in the TE signal and producing tracking abnormality. Consequently, some type of tracking failure such as a track jump happens. Various techniques have been proposed so far to avoid such tracking failures. For example, according to Japanese Patent Application Laid-Open Publication No. 2006-179136, the decrease in the intensity of the reflected light is detected on a regular basis, and if any defect, including the presence of a fingerprint, has been detected, the user is notified of that, thereby avoiding predictable write failures. Meanwhile, if the user permits such a soiled disc, a read/write operation will be performed on it just as instructed by him or her. Furthermore, by providing means for avoiding a repetitive notification, an opportunity to perform a read/write operation just as planned can be guaranteed while avoiding write failures to be caused by the dirt on the disc.
Various types of optical discs such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now.
To increase the storage density of an optical disc, the light beam that has been converged on the data plane of an optical disc preferably has as small a spot size as possible. The spot size of a light beam is inversely proportional to the numerical aperture NA of an objective lens for use to converge the light beam. Thus, by increasing the numerical aperture NA of the objective lens, the spot size of the light beam can be decreased. Meanwhile, Blu-ray Disc (BD) and other next-generation optical discs that have even higher storage density and even bigger storage capacity than the optical discs mentioned above have become more and more popular nowadays. A BD requires a larger objective lens NA than a DVD's and covers the surface of its information layer with a thin light transmitting layer with a thickness of approximately 100 μm. That is why compared to a DVD, a BD is more sensitive to dust, fingerprints and other sorts of defects because the light beam converged on the information layer of a BD is affected by such defects much more significantly than on a DVD.
In the prior art, not just the fingerprints but also various other defects such as dirt, grazes, scratches and dust on an optical disc are detected by sensing a decrease in the amplitude of a reflected light intensity (which will be referred to herein as an “RF amplitude”). That is to say, no matter what type of defect has been detected, the user is just notified of the presence of a defect on the disc. And he or she has no choice but to replace the disc with such a defect with another one.
However, in a situation where a scheduled recording operation is carried out while the user is away from home or after he or she has gone to bed at night with the recording schedule entered, even if the recorder tries to notify the user of the defect, he or she cannot replace the optical disc. In that case, the recorder cannot help either continuing the write operation on the optical disc, in which the defect has been detected, at the expense of the quality of the data stored, or stopping the write operation with an error message displayed.
Among other things, a BD is far more sensitive to scratches than a DVD is. Nevertheless, a BD is standardized as a bare optical disc. That is why even if only a tiny fingerprint were left accidentally on a brand-new BD that has just been removed from its case while it is loaded into a drive, a track jump or a write error would still be caused by the fingerprint.
What is worse, a fingerprint left on a BD could prevent the user from either writing data on it as intended or even reading data from it. As a matter of fact, results of experiments also revealed that a BD with a fingerprint had a significantly decreased RF amplitude and that a BD was much more sensitive to fingerprints than a DVD was. These results are quite understandable considering how a BD works in principle.
A fingerprint sometimes has as large a diameter as more than 10 mm. Thus, it is difficult to provide an alternative area that could substitute for the entire area with a fingerprint. In other words, if an area with a fingerprint were managed as a defective area without removing the fingerprint, the storage capacity of the optical disc would be wasted in vain. It has also been known that even if the area with the fingerprint is subjected to the same type of error correction processing as the one applied to a DVD, it is still difficult to correct the read/write errors that have been caused due to the presence of the fingerprint. That is why a BD has enhanced error correction ability. In addition, according to its initial standard (BD-RE: Ver. 1.0), a BD should be housed in a cartridge and non-removable from the cartridge to prevent the BD from being soiled with a fingerprint.
However, according to recent standards such as BD-RE: Ver. 2.0 and standards for BD-R and BD-ROM, a BD is supposed to be used as a bare optical disc (i.e., an optical disc without a cartridge) just like a CD or a DVD to meet the market needs. Thus, on the supposition that a fingerprint left on a BD should be wiped away manually with a cleaner such as a brush or a piece of cloth, the strength of the hard coating on the surface of a BD is defined by the Taber's scratch test. Furthermore, a cartridge in/from which a BD could be housed or removed is also defined to prevent the user from leaving a fingerprint on it.
Thanks to these standards and techniques, when a digital telecast is recorded on a BD or when a movie is played back from it, some errors could be caused due to the presence of a fingerprint or dust but would be just momentary noise that never persists long, thus achieving an acceptable level as far as a normal viewer's impression is concerned.
Meanwhile, in the field of personal computers (PCs), a user who uses a BD should be rather experienced and professional by now in handling various types of optical discs. And it is quite natural for such a person to handle a BD carefully enough to avoid spotting it with a fingerprint or dirt. More specifically, a PC user uses a BD in most cases to make a backup of data files that are stored on a hard disk. That is why if he or she is handling files with a PC by the drag-and-copy technique, for example, verification is automatically required after the files have been written. That is why if write errors were caused due to the presence of a fingerprint or dust, the verification would fail, thus letting the user know that the write errors have occurred. In that case, the user can avoid the write errors due to the presence of a fingerprint or dust by retrying writing a number of times until the verification succeeds.
As described above, various problems that could be caused by a fingerprint on a BD are not currently considered serious ones. However, the broader the applications of BDs and the greater the number of BD users, the more likely a fingerprint on a BD would raise a big issue. For example, to take advantage of the high storage density and big storage capacity of BDs fully, people are now exploring options to use them in industrial and medical applications. More specifically, banks may use them as archives to store a huge amount of customer data and market data. In medical fields, on the other hand, still pictures of CT scans and MRI, which have recently increased their definition significantly, and moving pictures shot with a gastric camera or an endoscope would be stored on BDs more and more often in the near future. In these applications, it is more strongly recommended to avoid those write errors one way or another.
More specifically, as for medical applications, it would be very difficult to remove an optical disc and clean it manually during a surgery or an inspection, for example. For that reason, in such applications, the dust or fingerprint should be cleanable automatically by a machine. Also, if the image data collected from a single patient is easily retrievable on a disc-by-disc basis (i.e., if the entire data about a single patient is managed on a single optical disc), then the person who is inspecting him or her can know more easily how much the disease has advanced, what will be the risks of complication in cases of surgery, or whatever has changed in him or her in general. In that case, the doctor can treat the patient more quickly and more appropriately without missing any little variation or symptom.
In that case, every time a patient is subjected to a surgery or inspection, high-definition moving pictures that have been shot with an endoscope or a gastric camera would be directly written on his or her dedicated optical disc without passing through any server computer or the hard disk drive of a PC. In such an application, even momentary noise is intolerable considering an affection that could be caused in the patient and the status of the diseased part of his or her parietal wall or gastric wall unlike the situation where a digital telecast needs to be recorded or a movie needs to be played as described above. That is why there will be an increasing demand for more reliable scheme and system than conventional ones.
Furthermore, as for car navigators, car BD players and other devices to be used in cars, a person who is driving a car is much more likely to leave a fingerprint on an optical disc to be newly loaded compared to a situation where a fixed recorder or player is used. According to a conventional technique, a defect, if any, is automatically detected during a disc loading process, and an alert “wipe the disc” is displayed to prompt the user to clean the disc. However, the same alert is also displayed even if the defect detected is just a graze or an uneven portion of the base member, not a fingerprint or dust, thus often making the user feel uncomfortable. What is more, the user cannot know exactly what part of the 12 cm disc needs to be wiped. That is to say, even if the user believes he or she has wiped the disc well, the wiped part does not always agree with the location where the fingerprint has actually been detected, and the same alert may be displayed over and over again.
In order to overcome the problems described above, the present invention dares to take advantage of the drawback of a BD that is more sensitive to fingerprints. And an object of the present invention is to provide an optical disc apparatus and an optical pickup with a cleaner that can notify the user exactly where on the optical disc a fingerprint has been detected, can wipe the fingerprint away automatically, and can transfer important data stored to an alternative location with no fingerprints by detecting the dots themselves of the fingerprint with a light beam spot very narrowly converged on the surface of the optical disc and locating the fingerprint accurately.
A more specific object of the present invention is to provide an optical disc apparatus and an optical pickup with a cleaner that ensure reliability that is high enough to use them even in industrial and medical applications with no problem.